Is the steppe migration theory of Indo-European origins correct after all?

One of the longest-running debates in the study of prehistory is the origin of the Indo-European language family. This group includes languages spoken from Great Britain and Ireland to India the steppes of Central Asia, and a connection between them was established as far back as the late eighteenth century. It is assumed that all originated from a single mother tongue, Proto-Indo-European (PIE), but where was PIE spoken?
Until the 1980s, the favoured Indo-European homeland was the Pontic-Caspian steppes north of the Black Sea and Caspian Sea. In a series of papers published from the 1950s through to the 1970s, Lithuanian-born Marija Gimbutas identified the Proto-Indo-Europeans with the Kurgan culture, named for a Russian word for the burial mounds with which the culture is associated. The Kurgan people originated in the lower Volga basin and the lower Dnieper region around 4500 BC. They lived as nomadic pastors, with an economy based on sheep, goats, cattle and pigs. Gimbutas saw them as highly-mobile, warlike people who used ox-drawn wagons and horses for transport. Between 4400 and 2800 BC, the Kurgan people mounted a series of hostile incursions from the steppes into Europe, Anatolia, the Caucasus, India and Central Asia. The peaceful, settled Neolithic farmers living in the regions were no match for the invaders, whose language (PIE) and culture came to predominate.

Although some disputed the warlike nature of the Kurgan people, the steppe hypothesis was widely accepted. However, in 1987, British archaeologist Colin Renfrew put forward a rival view in which he claimed that PIE was spoken and dispersed by Neolithic farmers originating from Anatolia before 6500 BC. The spread of agriculture distributed their language over a vast area. From Anatolia, the expansion had moved into Greece and onwards across Europe in wave of advance, expanding generation by generation as populations grew. Renfrew offered a choice of two scenarios as to the spread of the Indo-Iranian sub-family of languages: the first proposed a simple wave of advance similar to that proposed for Europe. The second invoked a modification of the steppe-invader model. Once the European wave of advance reached the steppe, nomadic pastoralism developed and the pastors moved swiftly east across the steppes and into Iran and northern India. Renfrew later came down in favour of this second scenario.

The Renfrew model is supported by archaeological and genetic evidence for an agricultural expansion into Europe, with Neolithic farmers both replacing and intermarrying with existing Mesolithic hunter-gatherers. Further support has come from a 2012 study in which Bayesian statistics were applied to Indo-European languages. The results suggested that PIE originated in Anatolia and began to break up into its daughter tongues between 7500 and 6000 BC.

Despite the success of the Anatolian hypothesis, it is not without its faults. A common criticism is that PIE contains reconstructed words for the wheel and wheeled vehicles, which were not invented until long after the agricultural expansion into Europe.

The main weakness of the steppe model has been a lack of evidence for a major migration from the Eurasian steppe at that time. However, a study of ancient DNA obtained from just under a hundred Europeans living between 6000 and 1000 BC may address this issue. The results confirm that farmers reached Europe from Southwest Asia between 6000 and 5000 BC, but they also indicate a second, later migration. A close match was found between Yamnaya culture steppe pastors from Russia and Ukraine, dating to 3000 BC, and individuals of the Corded Ware culture of Northern Europe, dating to 2500 BC. The similarities indicate a large-scale migration into Europe from the east. While the language these migrants spoke is unknown, it probably originated in the Yamnaya homeland.

Consistent with these results is a new linguistic study suggesting that PIE originated around 4000 BC rather than between 7500 and 6000 BC. However, the methodology used has been criticised by the authors of the 2012 study.

It is also possible that the Yamnaya data represents a secondary migration of people descended from the original PIE-speaking Anatolian farmers, in which case the Yamnaya people might have spoken a derived Indo-European language ancestral to the present-day Balto-Slavic group rather than PIE itself.

Overall, the genetic results suggest that the true picture might be a synthesis of both Anatolian and steppe-migration models.